1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same, and in particular, to a liquid crystal display device having an improved viewing angle characteristic and a method for producing the same.
2. Description of the Related Art
Conventionally, liquid crystal display (hereinafter, referred to as "LCD") devices using an electro-optical effect are known. The different types of LCD devices which have been put into practical use include, for example, twisted nematic LCD devices and super twisted nematic LCD devices, both of which use a nematic liquid crystal material. Hereinafter, twisted nematic LCD devices will be referred to as "TN LCD devices", and super twisted nematic LCD devices will be referred to as "STN LCD devices". LCD devices using a ferroelectric liquid crystal have also been proposed.
The above-mentioned LCD devices require a polarizing plate and some type of alignment treatment. In the initial state, the liquid crystal molecules are tilted at a pre-tilt angle in a uniform alignment direction while being twisted at an angle of 90.degree. in a liquid crystal layer. The liquid crystal molecules are treated using an alignment layer or the like so as to be erected in a uniform direction when a voltage of a prescribed level is applied. When such a voltage is applied as to display a half-tone image, the liquid crystal molecules are tilted at a certain angle in a uniform direction. Due to the tilting in the uniform direction and the birefringence of the liquid crystal material, the refractive index of the liquid crystal molecules is different when the liquid crystal molecules are seen from different directions. Due to the different refractive indices, the contrast of the image is significantly different depending on the direction from which the image is seen. In an extreme case, a defective display can occur such as display inversion in which the contrast of the displayed image is inverted. Thus, conventional LCD devices suffer from such drawbacks as a poor viewing angle characteristic.
Other types of LCD devices, which do not require polarizing plates, make use of a dynamic scattering (DS) effect or a phase change (PC) effect of a liquid crystal material.
Japanese Laid-Open Patent Publication No. 61-502128 describes still another type of LCD device which does not include a polarizing plate. This type of LCD device does not require alignment treatment either, and utilizes the birefringence of the liquid crystal to control the liquid crystal to be in a transparent state or an opaque state. Such an LCD device is basically operated in the following manner. A liquid crystal layer within the LCD device includes a liquid crystal and a polymer. The ordinary refractive index of the liquid crystal molecules and the refractive index of a polymer used as a surrounding are matched. Thus, when a voltage is applied to align the orientation directions of the liquid crystal molecules of the LCD device in the direction of the electric field, the liquid crystal layer is put into a transparent state to transmit light. When no voltage is applied, the orientation directions of the liquid crystal molecules are not uniform, namely, the liquid crystal layer is in an opaque state as a result of scattering the light. According to one known production method, such LCD devices are produced by mixing a photocurable resin or a thermosetting resin with a liquid crystal material and curing the resultant mixture to separate a liquid crystal phase from a polymer phase, thereby forming liquid crystal regions in the resin.
Japanese Laid-Open Patent Publication Nos. 4-338923 and 4-212928 describe a method for improving the viewing angle characteristic of a TN LCD device including a thin film transistor (hereinafter, referred to as the "TFT"). According to these publications, an LCD device to transmit the light in a transparent state or scatter the light in an opaque state includes a liquid crystal layer interposed between two polarizing plates, the polarizing axes of which are perpendicular to each other. In the method described in the above-mentioned publications, the light which is polarized by one of the polarizing plates is depolarized by the liquid crystal layer in the light scattering state. In principle, the brightness of the light incident on the LCD device is reduced down to 50% at least. In practice, the brightness of the light transmitted through this type of LCD devices is 1/3 of that transmitted through usual TN LCD devices (H. Yoshida, M. Ohashi and M. Okabe, Japan Display 1992, S-17, page 631).
Japanese Laid-Open Patent Publication No. 5-27242 describes another method for improving the viewing angle characteristic of a TN LCD device, in which a liquid crystal layer includes a plurality of liquid crystal regions separated from one another by a polymer wall. Each liquid crystal region includes a plurality of liquid crystal domains randomly arranged. The uniform arrangement of the liquid crystal domains is disturbed by the polymer wall, and thus the liquid crystal domains are put into a random state, thereby improving the viewing angle characteristic. However, in this method, the liquid crystal regions cannot be formed in areas accurately corresponding to a plurality of pixels arranged in a matrix. Because of this drawback, the transmittance of the light through the LCD device still suffers. Further, when the liquid crystal domains are arranged randomly, extreme deterioration, such as inversion, in the viewing angle characteristic does not occur. Nevertheless, there is still substantial light leakage in the LCD device when measured at an angle that is offset from a direction perpendicular to the substrate of the LCD device even during voltage saturation when ideally no light is transmitted.
In order to solve the above-described problems, researchers including some of the inventors of the present invention proposed another type of LCD device in Japanese Patent Application No. 5-30996. The LCD device described in this patent application includes a pair of substrates opposed to each other and a mixture of a liquid crystal, a photocurable resin and a photo-initiator injected between the substrates. A photomask is provided on a counter substrate included among the two substrates to substantially shield light from areas of the mixture corresponding to a plurality of pixels. When ultraviolet rays are radiated on the mixture from the side of the photomask, liquid crystal regions are formed in the areas in the mixture corresponding to the pixels, the areas being exposed to a very low intensity of the ultraviolet rays. The polymer aggregates in the other areas, namely, the areas which are not corresponding to the pixels and exposed to a high intensity of the ultraviolet rays. In such an LCD device, liquid crystal regions can be formed in areas corresponding to the pixels due to the photomask. In the case when, for example, a nematic liquid crystal is used to arrange the liquid crystal domains radially or randomly in the liquid crystal region, the viewing angle characteristic of the LCD device is significantly improved compared with that of usual TN LCD devices.
However, in the case when the counter substrate is also covered with a black mask having light transmitting holes at positions corresponding to the pixels, the ultraviolet rays are not transmitted through a sufficiently large portion of the LCD device to easily cure the photocurable resin.
In the case when the counter substrate having the photomask is also covered with a color filter, the ultraviolet rays are not transmitted through the color filter, and thus it is more difficult to cure the photocurable resin.
In order to avoid such problems, the photomask is provided on an active substrate opposed to the counter substrate, and the ultraviolet rays are radiated from the side of the active substrate through the photomask. However, since source lines and gate lines provided on the active substrate shield the ultraviolet rays, liquid crystal regions are formed below the source lines and the gate lines as well as in areas corresponding to the pixels. The formation of the liquid crystal regions below the source lines and the gate lines effects the composition of the liquid crystal material and polymer in the liquid crystal layer. More particularly, the polymer tends to remain in the areas corresponding to the pixels, and the liquid crystal regions formed below the source lines and the gate lines contact the liquid crystal regions formed in the areas corresponding to the pixels. As a result, the arrangement of the liquid crystal domains in the liquid crystal regions formed in the areas corresponding to the pixels is adversely affected, thereby reducing the viewing angle characteristic of the LCD device.